Genes encoding plant transcription factors

ABSTRACT

Identification of a gene from monocotyledonous plants such as rice, which codes for a transcription factor specific to a stress tolerant gene and provision of a novel environmental stress tolerant plant using the gene. From the rice genome, a gene, which binds to a cis element existing upstream of the gene encoding a stress responsive protein and for a transcription factor to activate the transcription of the gene, is identified. Further, the gene of the transcription factor is used to transform a plant, thereby improving tolerance against environmental stresses such as low temperature, dehydration, and salt stresses.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a protein, which regulates rice-derived environmental stress tolerance, a gene encoding the same, and a method for utilizing the same.

2. Prior Art

Plants possess tolerance mechanisms to cope with various types of environmental stresses in nature such as dehydration, high temperature, freezing, or salt stress. In the production of plants having such environmental stress tolerance, techniques have been heretofore used for genetically selecting and mating strains which are dehydration, salt, or low temperature tolerant. However, these techniques require long periods of time to select, and also have low success rates.

On the other hand, as the stress tolerance mechanism is elucidated at a molecular level, stress tolerant plants have been produced using biotechnological techniques. For example, it has been shown that stress proteins such as LEA proteins, water channel proteins, or synthetases for compatible solutes are induced in cells when they are exposed to stress, thereby protecting the cells from such stress. Thus, research has been attempted in which genes such as LEA proteins of barley or detoxification enzymes of tobacco, or genes of synthetases for osmoregulatory substances (e.g., sugar, proline, or glycinebetaine) are introduced into host plants. Research using genes encoding w-3 fatty acid desaturase of Arabidopsis thaliana, the D9-desaturase of blue-green algae, or the like, which are modification enzymes of the cellular membrane lipid, has also been attempted. In the above researches, a gene was bound to the 35S promoter of cauliflower mosaic virus and introduced into a plant. The level of stress tolerance of the recombinant plant was, however, low and unstable. Thus, none of these was put to practical use.

On the other hand, stress tolerance mechanism is found to be intricately associated with several genes (Plant Physiol., 115: 327-334 (1997)). Accordingly, research in which a gene encoding a transcription factor which simultaneously activates the expression of the genes is introduced into a plant, thereby enhancing the plant's stress tolerance, has been attempted (The Plant Cell, 10: 1-17 (1998)). However, when several genes are simultaneously activated, the energy of the host plant becomes directed towards the generation of the gene product or intracellular metabolism resulting from the gene product. Accordingly, the growth of the plant itself deteriorates or becomes retarded.

In contrast, the present inventors had isolated the genes DREB1A, DREB1B, DREB1C, DREB2A, and DREB2Bencoding the transcription factors which bind to a stress responsive element and specifically activate the transcription of genes located downstream of the element from Arabidopsis thaliana (Japanese Patent Application Laying-Open (kokai) No. 10-228457). They reported that introduction and overexpression of the genes in a plant enabled impartment of stress tolerance without causing retardation of a plant (Japanese Patent Application Laying-Open (kokai) 10-292348).

Arabidopsis thaliana is classified as a dicotyledonous plant while major crops such as rice, maize, and wheat are classified as monocotyledonous plants. Dicotyledonous plants are relatively different from monocotyledonous plants from the viewpoint of plants evolution. It has been shown that the DREB1A gene of Arabidopsis thaliana functions well in monocotyledonous plants, but not as well in dicotyledonous plants. Thus, if a DREB-homologous gene derived from the monocotyledonous plant can be isolated, environmental stress tolerance can be more efficiently transmitted to monocotyledonous plants thereby.

SUMMARY OF THE INVENTION

An object of the present invention is to identify from a monocotyledonous plant such as rice, a gene which codes for a transcription factor specific to a stress tolerant gene, and to provide a novel environmental stress tolerant plant using the same gene.

The present inventors have conducted concentrated studies in order to attain the above object. As a result, they had succeeded in identifying all the DREB-homologous genes from the rice genome. They also found that introduction of the genes into other plants significantly enhanced their environmental stress tolerance. This has led to the completion of the present invention.

More specifically, the present invention provides the following (1) to (12).

(1) An isolated gene comprising the following DNA (a) or (b):

-   -   (a) DNA which comprises the nucleotide sequence as shown in SEQ         ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, or SEQ ID         NO: 9; or     -   (b) DNA which hybridizes with the DNA comprising a nucleotide         sequence, which is complementary to the DNA comprising the         nucleotide sequence as shown in SEQ ID NO: 1, SEQ ID NO: 3, SEQ         ID NO: 5, SEQ ID NO: 7, or SEQ ID NO: 9 under stringent         conditions and which codes for a protein that regulates the         transcription of genes located downstream of a stress responsive         element.

(2) An isolated gene encoding the following protein (c) or (d):

-   -   (c) a protein which comprises the amino acid sequence as shown         in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or         SEQ ID NO: 10; or     -   (d) a protein which comprises the amino acid sequence having         deletion, substitution, or addition of one or several amino         acids in the amino acid sequence as shown in SEQ ID NO: 2, SEQ         ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10 and which         regulates the transcription of genes located downstream of a         stress responsive element.

(3) The gene according to (1) or (2) above, wherein the stress is dehydration stress, low temperature stress, or salt stress.

(4) The following recombinant protein (c) or (d):

-   -   (c) a protein which comprises the amino acid sequence as shown         in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or         SEQ ID NO: 10; or     -   (d) a protein which comprises the amino acid sequence having         deletion, substitution, or addition of one or several amino         acids in the amino acid sequence as shown in SEQ ID NO: 2, SEQ         ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10 and which         regulates the transcription of genes located downstream of a         stress responsive element.

(5) The protein according to (4) above, wherein the stress is dehydration stress, low temperature stress, or salt stress.

(6) A recombinant vector comprising the gene according to any one of (1) to (3) above.

(7) A transformant transformed with the recombinant vector according to (6) above.

(8) The transformant according to (7) above, wherein the host is a plant.

(9) The transformant according to (7) above, wherein the host is a monocotyledonous plant.

(10) A method for producing a protein which regulates the transcription of genes located downstream of a stress responsive element, wherein the transformant according to (8) or (9) above is cultured in a medium and the protein is recovered from the resultant culture product.

(11) A method for determining stress levels in plants, wherein the transcription levels of the gene according to any one of (1) to (3) above in plant bodies are determined.

(12) A method for improving the stress tolerance of plants by introducing the gene according to any one of (1) to (3) above into the plants.

The present invention provides a stress tolerant transcription factor derived from monocotyledonous plants and a gene encoding this transcription factor. Use of the gene according to the present invention enables the more efficient transmission of stress tolerance to crops, i.e., monocotyledonous plants.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawings execute din color. Copies of this patent or patent application publication with color drawings will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 shows a structure of the OsDREB protein (A: OsDREB1, B: OsDREB2).

FIG. 2 shows an amino acid sequence of a DREB1-homologous protein (OsDREB1A, OsDREB1B, OsDREB1C, OsDREB1D; rice, BCBF3; barley, DREB1A; Arabidopsis thaliana, ACRE111B; tobacco).

FIG. 3 shows an amino acid sequence of a DREB2-homologous protein (OsDREB2A; rice, DREB2A: Arabidopsis thaliana, ORCA1; Catharanthus roseus).

FIG. 4 shows the result of a gel shift assay.

FIG. 5(A) shows the structure of a plasmid used in transactivation, and FIG. 5(B) shows a ratio of introduction efficiency by transactivation (GUS/LUC).

FIG. 6 shows the result of Northern blotting analysis of expression of OsDREB genes.

FIG. 7 shows the result of analysis of stress tolerant gene expression in a recombinant Arabidopsis thaliana.

FIG. 8 shows the result of analysis of stress tolerant gene expression in a recombinant rice.

FIG. 9 shows salt stress tolerance of a recombinant plant (Arabidopsis thaliana) into which OsDREB1A and DREB1A have been introduced.

DETAILED DESCRIPTION OF THE INVENTION

This specification includes part or all of the contents as disclosed in the specification of Japanese Patent Application No. 2001-358268, which is a priority document of the present application.

The gene according to the present invention is a gene derived from rice genome having tolerance improving mechanisms against environmental stresses such as low temperature, dehydration, or salt stress.

The gene of the present invention is “an isolated gene encoding a transcription factor which binds to a cis element located upstream of genes encoding stress responsive proteins expressed in response to environmental stresses such as low temperature, dehydration, or salt stress, thereby activating the transcription of the genes”. Specific examples of the above cis element include dehydration-responsive element (DRE), abscisic acid-responsive element (ABRE), and low temperature-responsive element. The protein encoded by the gene of the present invention functions to activate the transcription of genes located downstream of the above-mentioned stress responsive elements (DRE or the like).

The gene according to the present invention can be identified as, for example, described below.

1. Identification of the Gene of the Present Invention

The gene according to the present invention can be screened based on homology with a known gene having homologous functions, that is, a gene encoding a transcription factor specific to a stress tolerant gene of a plant. mRNA and cDNA libraries of rice or a rice genomic library may be prepared and may be subjected to screening. Alternatively, an existing database of rice DNA may be subjected to screening.

A. Screening of Gene Library

(1) Preparation of mRNA and cDNA Libraries

At the outset, mRNA and cDNA libraries are prepared as follows.

As a source of mRNA, parts of the plant body of rice such as leaves, stems, roots, or flowers, or the plant body as a whole may be used. Alternatively, a plant body obtained by sowing rice seeds on a solid medium such as GM medium, MS medium, or #3 medium and growing them aseptically may be used. The source may be a callus or a cultured rice cell which was aseptically grown, and the variety thereof is not particularly limited as long as the cell contains mRNA of the gene of interest. Further, since the gene to be screened is expressed in response to environmental stress, plants that are exposed to low temperature stress (e.g. 10 to −4° C.), salt stress (e.g. 150 to 250 mM NaCl), or dehydration stress (e.g. dehydrated state) can also be preferably used.

For example, mRNA is prepared as follows. Rice plant bodies, which had been grown hydroponically to low temperature stress, dehydration stress, or salt stress are exposed and then freezed with liquid nitrogen. The frozen plant bodies are ground in a mortar. From the resultant ground material, crude RNA fraction is extracted and prepared by the glyoxal method, the guanidine thiocyanate-cesium chloride method, the lithium chloride-urea method, the proteinase K-deoxyribonuclease method, or the like. From this crude RNA fraction, poly(A)+ RNA (mRNA) can be then obtained by the affinity column method using oligo dT-cellulose or poly U-Sepharose carried on Sepharose 2B or by the batch method. The resultant mRNA may further be fractionated by sucrose density gradient centrifugation or the like.

Further, a cDNA library can be produced using the thus obtained mRNA as a template. For example, single-stranded cDNA is synthesized using an oligo(dT) primer or random primer, and a reverse transcriptase in a commercially available kit (e.g. ZAP-cDNA Synthesis Kit: Stratagene). Then, double-stranded cDNA is synthesized from the resultant single-stranded cDNA. Subsequently, an adaptor containing a suitable restriction site is added to the resultant double-stranded cDNA, which is then inserted into a cloning site of a lambda phage vector. The resultant DNA is packaged using Gigapack III Gold packaging extract (Stratagene) or the like and infected into an E. coli host, and then amplified. Thereafter, phage particles are recovered and stored.

(2) Preparation of Genomic Library

For example, the preparation of a genomic library using a lambda phage vector is carried out in the following manner. As a source of DNA, parts of the rice plant body such as leaves, stems, roots, or flowers, or the plant body as a whole may be used as long as the tissue contains DNA. The plant body is pulverized in the presence of liquid nitrogen, and DNA is extracted by the CTAB method, the benzyl chloride method, or the like. The resultant DNA is partially decomposed with the restriction enzyme Sau3AI and then fractionated by NaCl density gradient ultracentrifugation or the like to recover 10 to 20 kb fragments. These fragments are inserted into the BamHI cleavage site of lambda phage vectors such as λEMBL3 and λFIX II. Thereafter, packaging is carried out using Gigapack III Gold packaging extract (Stratagene) or the like, followed by infection into an E. coli host. The amplified phage particles are then recovered and stored.

(3) Screening of Library

A library can be screened in the following manner.

A DNA fragment as a probe is prepared based on a sequence in a highly conserved region of, for example, a known gene encoding a transcription factor specific to a stress tolerant gene of a plant, such as the DREB gene derived from Arabidopsis thaliana (DREB1A gene: SEQ ID NO: 11, DREB2A gene: SEQ ID NO: 12, DREB1B gene: SEQ ID NO: 13, DREB1C gene: SEQ ID NO: 14, DREB2B gene: SEQ ID NO: 15). The probe DNA may be amplified by PCR using two primers with approximately 15 bp to 25 bp which are designed based on the sequence of each side of the highly conserved region so as to amplify said region. When the highly conserved region is short and insufficient as a probe, a primer may be designed to amplify several highly conserved regions together with the regions adjacent thereto.

Using the above probe, a cDNA library or genomic library is screened by plaque hybridization or colony hybridization.

B. Screening Using Gene Database

Important sequences (highly conserved regions or regions deduced to have desired function) of, for example, a known gene that encodes a transcription factor specific to a stress tolerant gene of a plant such as DREB gene derived from Arabidopsis thaliana (DREB1A gene, DREB1B gene, DREB1C gene, DREB2A genes, DREB2B gene) are specified. Subsequently, homology search on an existing gene database is conducted based on the specified sequence. The genetic data to be searched may be EST or a full-length gene. Homology search can be carried out using an analytical software such as BLAST or FASTA on databases of GenBank or DDBJ. Preferably, the object of detection is a gene encoding an amino acid sequence which has an especially high homology with an amino acid sequence in the highly conserved region or a region deduced to have desired function, consequently a sequence conserving an amino acid sequence that is essential for the function of a protein. Based on the resultant sequence, a primer is designed, and PCR is carried out using uncloned cDNA (RT-PCR), a cDNA library, genomic DNA, or a genomic library as a template, thereby obtaining the gene of interest. Alternatively, a DNA fragment amplified by PCR is used as a probe and a cDNA library or genomic library is screened to obtain the gene of interest.

C. Determination of Nucleotide Sequences

The entire nucleotide sequence of the cloned cDNA can be determined in accordance with conventional methods. Nucleotide sequencing includes the chemical modification method of Maxam-Gilbert or the dideoxynucleotide chain termination method using M13 phage. Usually, sequencing is carried out using an automated nucleotide sequencer (e.g., 377 DNA Sequencer, Perkin-Elmer).

Thus, OsDREB1A (SEQ ID NO: 1), OsDREB1B (SEQ ID NO: 3), OsDREB1C (SEQ ID NO: 5), OsDREB1D (SEQ ID NO: 7), and OsDREB2A (SEQ ID NO: 9) were identified as DREB-homologous genes derived from rice.

Also, OsDREB1A protein (SEQ ID NO: 2), OsDREB1B protein (SEQ ID NO: 4), OsDREB1C protein (SEQ ID NO: 6), OsDREB1D protein (SEQ ID NO: 8), and OsDREB2A protein (SEQ ID NO: 10), which were coded by the genes through analysis of ORFs of the genes, were identified.

The genes according to the present invention, however, are not limited to genes comprising DNA as shown in SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, or SEQ ID NO: 9. Genes comprising DNA, which are hybridizable under stringent conditions with DNA comprising a nucleotide sequence that is complementary to the DNA comprising a nucleotide sequence as shown in SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, or SEQ ID NO: 9, are also genes of the present invention as long as they code for proteins that regulate the transcription of genes located downstream of a stress responsive element.

The term “stringent conditions” as used herein refers to parameters with which the art is familiar. Stringent conditions are sequence-dependent and will be different in different circumstances. Longer sequences hybridize specifically at higher temperatures. Generally, stringent conditions are selected to be about 5° C. lower than the thermal melting point (T_(m)) for the specific sequence at a defined ionic strength and pH. The T_(m) is the temperature (under defined ionic strength, pH and nucleic acid concentration) at which 50% of the probes complementary to the target sequence hybridize to the target sequence at equilibrium. Nucleic acid hybridization parameters may be found in references which compile such methods, e.g., Molecular Cloning: A Laboratory Manual, J. Sambrook, et al., eds., Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1989. More specifically, stringent conditions, as used herein, refers, for example, to those conditions in which formamide concentration is 30-50%, temperature is 37 to 50° C., and 6×SSC. Preferably, formamide concentration is 50%, temperature is 42° C., and 6×SSC.

The genes of the present invention are genes encoding proteins comprising amino acid sequences as shown in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10. Even though a protein comprises an amino acid sequence having deletion, substitution, or addition of one or several amino acids in the amino acid sequence as shown in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10, genes encoding this protein is included as genes according to the present invention as long as this protein can regulate the transcription of genes located downstream of a stress responsive element. The term “several amino acids” preferably refers to 20 or fewer and more preferably 5 or fewer amino acids.

The protein according to the present invention is not limited to a protein comprising an amino acid sequence as shown in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10. A protein which comprises an amino acid sequence having one or several amino acids deleted, substituted, or added in the amino acid sequence as shown in SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, or SEQ ID NO: 10 is included as a protein according to the present invention as long as it can regulate the transcription of genes located downstream of a stress responsive element. The term “several amino acids” preferably refers to 20 or fewer and more preferably 5 or fewer amino acids.

The introduction of mutation into the gene of the present invention may be performed by conventional techniques such as the Kunkel method, the Gapped duplex method or variations thereof using a mutation introducing kit [e.g. Mutant-K (Takara) or Mutant-G (Takara)] utilizing site-directed mutagenesis or using an LA PCR in vitro Mutagenesis Series Kit (Takara).

Once the nucleotide sequence for the gene of the present invention has been determined, the gene of the present invention can be obtained either by chemical synthesis, by PCR using the cDNA or genomic DNA of the gene as a template, or by the hybridization of a DNA fragment having the above nucleotide sequence as a probe.

2. Analysis of the DRE Binding Ability and Transcription Activating Ability of the Proteins of the Present Invention

A. Analysis of the DRE Binding Ability

The ability of the protein according to the present invention to bind to DRE can be confirmed by gel shift assay [Urao, T. et al., Plant Cell 5:1529-1539 (1993)] using a fusion protein composed of the protein, GST, and the like. The protein according to the present invention can be prepared by ligating the gene according to the present invention downstream of the glutathione-S-transferase (GST) coding region of a plasmid coding for GST gene (e.g. pGEX-4T-1 vector: Pharmacia) in a manner that the reading frames of the two genes coincide with each other, culturing E. coli that has been transformed with the plasmid under conditions which induce synthesis of the fusion protein, and purifying the protein from the transformed E. coli.

Gel shift assay is a method for examining the interaction between DNA and a protein. A DRE-containing DNA fragment labeled with ³²P or the like is mixed with the fusion protein described above and incubated, and the resultant mixture is subjected to electrophoresis. After drying, the gel is autoradiographed to detect those bands which have migrated to the back as a result of the binding of the DNA fragment and the protein. The specific binding of the protein according to the present invention to the DRE sequence can be confirmed by showing that the above-mentioned band is not detected when a DNA fragment containing a mutated DRE sequence is used.

B. Analysis of Transcription Activating Ability

The transcription activating ability of the proteins of the present invention can be analyzed by a transactivation experiment using rice protoplast system. For example, OsDREB1A cDNA is ligated to pBI221 plasmid (Clontech) containing CaMV35 S promoter to construct an effector plasmid. On the other hand, the DRE-containing DNA fragment is ligated upstream of TATA promoter located upstream of a β-glucuronidase (GUS) gene to construct a reporter plasmid. Subsequently, these two plasmids are introduced into rice protoplasts and then GUS activity is measured. If GUS activity is increased by the simultaneous expression of OsDREB1A protein, it is understood that OsDREB1A protein expressed in the protoplasts is activating the transcription through the DRE sequence.

In the present invention, preparation of protoplasts and introduction of plasmid DNA into the protoplasts may be performed by the method of Abel et al. [Abel, S. et al., Plant J. 5:421-427 (1994)]. In order to minimize experimental errors resulting from differences in plasmid DNA introduction efficiencies, a plasmid in which luciferase gene is ligated downstream of CaMV35S promoter may be introduced to protoplasts together with the two plasmids described above, thus, β-glucuronidase activity against luciferase activity may be determined. Then, the determined value may be taken to indicate transcription activating ability. β-glucuronidase activity can be determined by the method of Jefferson et al. [Jefferson, R. A. et al., EMBO J. 83:8447-8451 (1986)]; and luciferase activity can be determined using PicaGene Luciferase Assay Kit (Toyo Ink).

3. Preparation of Recombinant Vectors and Transformants

A. Preparation of Recombinant Vectors

The recombinant vector of the present invention can be obtained by ligating (inserting) the gene of the present invention to (into) an appropriate vector. The vector into which the gene of the present invention is to be inserted is not particularly limited as long as it is replicable in a host. For example, plasmid DNA, phage DNA or the like may be used. Plasmid DNA includes plasmids for E. coli hosts such as pBR322, pBR325, pUC118, and pUC119; plasmids for Bacillus subtilis hosts such as pUB110 and pTP5; plasmids for yeast host such as YEp13, YEp24, and YCp50; and plasmids for plant cell host such as pBI221 and pBI121. Phage DNA includes λ phage and the like. Further, animal virus vector such as retrovirus or vaccinia virus; or insect virus vector such as baculovirus may also be used.

In order to insert the gene of the present invention into a vector, for example, a method may be employed in which the purified DNA is cleaved with an appropriate restriction enzyme and then inserted into the restriction site or the multi-cloning site of an appropriate vector DNA for ligation to the vector. The gene of the present invention should be incorporated into the vector in such a manner that the function of the gene is expressed. For this purpose, in addition to a promoter and the gene of the present invention, those containing cis elements such as enhancer, a splicing signal, poly(A) addition signal, selection marker, ribosome binding sequence (SD sequence) or the like can be ligated to the vector of the present invention, if so desired. Examples of selection marker are dihydrofolate reductase gene, ampicillin tolerance gene, neomycin tolerance gene, or the like.

B. Preparation of Transformants

The transformant of the present invention can be obtained by introducing the recombinant vector of the present invention into a host so that the gene of interest can be expressed. The host is not particularly limited as long as the gene of the present invention can be expressed therein. Specific examples of the host include Escherichia bacteria such as E. coli; Bacillus bacteria such as Bacillus subtilis; Pseudomonas bacteria such as Pseudomonas putida; Rhizobium bacteria such as Rhizobium meliloti; yeasts such as Saccharomyces cerevisiae, Schizosaccharomyces pombe; plant cell strains established from Arabidopsis thaliana, tobacco, maize, rice, carrot, etc. or protoplasts prepared from such plants; animal cells such as COS cells, CHO cells; or insect cells such as Sf9 cells and Sf21 cells.

When a bacterium such as E. coli is used as the host, the recombinant vector of the present invention is capable of autonomous replication inside the host and, at the same time, it is preferably composed of a promoter, a ribosome binding sequence, the gene of the present invention, and a transcription termination sequence. The vector may also contain a gene to regulate the promoter. Escherichia coli strains such as HMS174 (DE3), K12, or DH1 may be used. Bacillus subtilis strains such as MI 114 or 207-21 may be used.

Any promoter may be used as long as it is able to direct the expression of the gene of interest in a host such as E. coli. For example, an E. coli- or phage-derived promoter such as trp promoter, lac promoter, P_(L) promoter, or P_(R) promoter may be used. An artificially designed and altered promoter such as tac promoter may also be used. Methods for introducing the recombinant vector into a bacterium are not particularly limited, and examples thereof include a method using calcium ions [Cohen, S. N. et al., Proc. Natl. Acad. Sci., USA, 69:2110-2114 (1972)] and electroporation.

When yeast such as Saccharomyces cerevisiae, Schizosaccharomyces pombe, or Pichia pastoris is used as the host, the promoter is not particularly limited, and any promoter may be used as long as it is able to direct the expression of the gene of interest in yeast. For example, gal10 promoter, gal 10 promoter, heat shock protein promoter, MFα1 promoter, PH05 promoter, PGK promoter, GAP promoter, ADH promoter, or AOX1 promoter can be used.

A method for introducing the recombinant vector into yeast is not particularly limited, and examples thereof include electroporation [Becker, D. M. et al., Methods Enzymol., 194:182-187 (1990)], the spheroplast method [Hinnen, A. et al., Proc. Natl. Acad. Sci., USA, 75:1929-1933 (1978)], and the lithium acetate method [Itoh, H., J. Bacteriol., 153:163-168 (1983)].

When a plant cell is used as the host, for example, cell strains established from rice, maize, wheat, Arabidopsis thaliana, tobacco, carrot, etc. or protoplasts prepared from such plants, the promoter to be used is not particularly limited as long as it is able to direct the expression of the gene of interest in plants. Examples thereof include 35S RNA promoter of cauliflower mosaic virus, rd29A gene promoter, and rbcS promoter.

A method for introducing the recombinant vector into a plant includes the method of Abel et al. using polyethylene glycol [Abel, H. et al., Plant J. 5:421-427 (1994)] and electroporation. When an animal cell is used as the host, for example, simian COS-7 or Vero cells, Chinese hamster ovary cells (CHO cells), mouse L cells, rat GH3 cells, human FL cells, or the like, SRα promoter, SV40 promoter, LTR promoter, CMV promoter or the like may be used. The early gene promoter of human cytomegalovirus or the like may also be used.

To introduce the recombinant vector into an animal cell, for example, electroporation, the calcium phosphate method, lipofection, or the like may be used. When an insect cell is used as the host, for example, Sf9 cells, Sf21 cells, or the like, the calcium phosphate method, lipofection, electroporation, or the like may be used.

4. Production of the Proteins According to the Present Invention

The protein of the present invention is a protein having an amino acid sequence encoded by the gene of the present invention; or a protein which has an amino acid sequence having at least one amino acid mutation in the above-described amino acid sequence and is able to regulate the transcription of genes located downstream of a stress responsive element.

The protein of the present invention can be obtained by culturing the transformant in a medium and recovering the protein from the resultant culture product. The term “culture product” means any of the following materials: culture supernatant, cultured cells, cultured microorganisms, or disrupted cells or microorganisms. The transformant of the present invention in a medium is cultured by conventional methods for culturing a host.

As a medium for culturing the transformant obtained from a microorganism host such as E. coli or yeast, either a natural or synthetic medium may be used as long as it contains carbon sources, nitrogen sources, and inorganic salts assimilable by the microorganism and is capable of efficient culture of the transformant. When a plant cell is used as the host, vitamins such as thiamine and pyridoxine can be added to the medium, if necessary. When an animal cell is used as the host, serum such as RPMI1640 can be added to the medium, if necessary.

Examples of carbon sources include: carbohydrates such as glucose, fructose, sucrose, and starch; organic acids such as acetic acid and propionic acid; and alcohols such as ethanol and propanol. Examples of nitrogen sources include: ammonia; ammonium salts of inorganic or organic acids such as ammonium chloride, ammonium sulfate, ammonium acetate, and ammonium phosphate; other nitrogen-containing compounds; peptone; meat extract; and corn steep liquor.

Examples of inorganic substances include: monopotassium phosphate, dipotassium phosphate, magnesium phosphate, magnesium sulfate, sodium chloride, iron(I) sulfate, manganese sulfate, copper sulfate, and calcium carbonate. Usually, the culture is carried out under aerobic conditions (such as shaking culture or aeration agitation culture) at approximately 30 to 37° C. for approximately 6 hours to 3 days. During the culture, the pH is maintained at approximately 7.0 to 7.5. The pH is adjusted with an inorganic or organic acid, an alkali solution, or the like.

During the culture, an antibiotic such as ampicillin or tetracycline may be added to the medium, if necessary. When a microorganism transformed with an expression vector containing an inducible promoter is cultured, an inducer may be added to the medium, if necessary. For example, when a microorganism transformed with an expression vector containing Lac promoter is cultured, isopropyl-β-D-thiogalactopyranoside (IPTG) or the like may be added to the medium. When a microorganism transformed with an expression vector containing trp promoter is cultured, indoleacrylic acid (IAA) or the like may be added to the medium.

Usually, the culture is carried out in the presence of 5% CO₂ at approximately 30 to 37° C. for approximately 6 hours to 3 days. During the culture, an antibiotic such as kanamycin or penicillin may be added to the medium if necessary. After the culture, the protein of the present invention is extracted by disrupting the cultured microorganism or cell if the protein is produced in the microorganism or cell. If the protein of the present invention is secreted outside of the microorganism or cell, the culture fluid may be used for the following steps as it is or subjected to centrifugation to remove the microorganism or cells. Thereafter, conventional biochemical techniques used for isolating/purifying a protein, for example, ammonium sulfate precipitation, gel chromatography, ion exchange chromatography, and affinity chromatography, are employed independently or in an appropriate combination to isolate and purify the protein of the present invention from the above culture product.

5. Preparation of Transgenic Plants into which the Gene of the Present Invention has been Introduced

A transgenic plant tolerant to environmental stresses, in particular, low temperature, freezing, and dehydration stresses, can be produced by introducing DNA encoding the protein of the present invention into a host plant using genetic engineering techniques. A method for introducing the gene of the present invention into a host plant includes indirect introduction such as the Agrobacterium infection method and direct introduction such as the particle gun method, polyethylene glycol method, liposome method, and microinjection method. When the Agrobacterium infection method is used, the transgenic plant of the present invention can be produced by the following procedure.

A. Preparation of a Recombinant Vector to be Introduced into a Plant and Transformation of Agrobacterium

A recombinant vector to be introduced into a plant can be prepared by cleaving with an appropriate restriction enzyme DNA comprising the genes of the present invention, ligating an appropriate linker to the resultant DNA if necessary, and inserting the DNA into a cloning vector for the plant cell host. A binary vector type plasmid such as pBI2113Not, pBI2113, pBI101, pBI121, pGA482, pGAH, and pBIG, or an intermediate vector type plasmid such as pLGV23Neo, pNCAT, and pMON200 may be used as cloning vectors.

When a binary vector type plasmid is used, the gene of interest is inserted between the border sequences (LB, RB) of the binary vector. The resultant recombinant vector is amplified in E. coli. The amplified recombinant vector is then introduced into Agrobacterium tumefaciens C58, LBA4404, EHA101, C58C1Rif^(R), EHA105, etc. by freeze-thawing, electroporation, or the like. The resultant Agrobacterium is used for the transformation of the plant of interest.

In the present invention, the three-member conjugation method [Nucleic Acids Research, 12:8711 (1984)] may also be used in addition to the method described above to prepare an Agrobacterium containing the gene of the present invention for plant infection. Specifically, plasmid-containing E. coli comprising the gene of interest, helper plasmid-containing E. coli (e.g. pRK2013), and an Agrobacterium are mixed and cultured on a medium containing rifampicin and kanamycin. Thus, a zygote Agrobacterium for infecting plants can be obtained.

For the expression of a foreign gene and the like in a plant body, a promoter and a terminator for plants should be located upstream and downstream of the structural gene, respectively. Specific examples of promoters which may be utilized in the present invention include cauliflower mosaic virus (CaMV)-derived 35S transcript [Jefferson, R. A. et al., The EMBO J. 6:3901-3907 (1987)]; the promoter for maize ubiquitin gene [Christensen, A. H. et al., Plant Mol. Biol. 18:675-689 (1992)]; the promoter for nopaline synthase (NOS) gene; and the promoter for octopin (OCT) synthase gene. Specific examples of useful terminator include CaMV-derived terminators and NOS-derived terminators. Promoters and terminators are not limited to the above-mentioned as long as they are known to function in plant bodies.

If the promoter used in a transgenic plant is a promoter responsible for the constitutive expression of the gene of interest (e.g., CaMV 35S promoter) and the use thereof has brought about delay in the growth or retardation of the transgenic plant, a promoter which directs transient expression of the gene of interest (e.g., rd29A gene promoter) may be used. If necessary, an intron sequence, which enhances the expression of the gene of the present invention, may be located between the promoter sequence and the gene. For example, the intron from maize alcohol dehydrogenase (Adh1) [Genes & Development 1:1183-1200 (1987)] may be introduced.

In order to efficiently select transformed cells of interest, it is preferable to use an effective selection marker gene in combination with the gene of the present invention. As the selection marker, one or more genes, which are selected from kanamycin tolerance (NPTII) gene, hygromycin phosphotransferase (htp) gene which confers tolerance to the antibiotic hygromycin on plants, phosphinothricin acetyl transferase (bar) gene which confers tolerance to bialaphos, and the like, can be used. The gene of the present invention and the selection marker gene may be incorporated together into a single vector. Alternatively, two types of recombinant DNAs may be used which are incorporated into separate vectors.

B. Introduction of the Gene of the Present Invention into a Host

In the present invention, while the host for the transformant is not particularly limited, it is preferably a plant. The plant may be any cultured plant cells, the entire plant body of a cultured plant, plant organs (such as leaves, petals, stems, roots, rhizomes, or seeds), or plant tissues (such as epidermis, phloem, parenchyma, xylem, or vascular bundle). Plants are preferably monocotyledonous plants such as rice, maize, and wheat. When a cultured plant cell, plant body, plant organ or plant tissue is used as the host, the Agrobacterium infection method, particle gun method, or polyethylene glycol method can be employed to introduce the DNA encoding the protein of the present invention to transform this host plant by introducing a vector into plant sections. Alternatively, a vector can be introduced into a protoplast by electroporation to produce a transformed plant.

For example, when a gene is introduced into Arabidopsis thaliana by the Agrobacterium infection method, the step of infecting the plant with an Agrobacterium containing a plasmid comprising the gene of interest is essential. This step can be performed by the vacuum infiltration method [CR Acad. Sci. Paris, Life Science, 316:1194 (1993)]. Specifically, Arabidopsis thaliana is grown in a soil composed of equivalent portions of vermiculite and perlite. The Arabidopsis thaliana is immersed directly in a culture fluid of an Agrobacterium containing a plasmid comprising the gene of the present invention, placed in a desiccator, and then sucked with a vacuum pump to 65-70 mmHg. Then, the plant is allowed to stand at room temperature for 5-10 min. The plant pot is transferred to a tray, which is covered with a wrap to maintain humidity. On the next day, the wrap is removed. The plant is grown in that state to harvest seeds.

Subsequently, the seeds are sown on MS agar medium supplemented with appropriate antibiotics to select those individuals which have the gene of interest. Arabidopsis thaliana grown on this medium are transferred to pots and grown there. As a result, seeds of a transgenic plant into which the gene of the present invention has been introduced can be obtained. Generally, the genes are introduced into the genome of the host plant in a similar manner. However, due to differences in the specific locations on the genome into which the genes have been introduced, the expression of the introduced genes varies. This phenomenon is called “position effect.” By assaying transformants with DNA fragments from the introduced gene as a probe by Northern blotting, it is possible to select those transformants in which the introduced gene is expressed more highly.

The confirmation that the gene of interest is integrated in the transgenic plant into which the gene of the present invention has been introduced and in the subsequent generation thereof can be made by extracting DNA from cells and tissues of those plants and detecting the introduced gene by PCR or Southern analysis, which are conventional methods in the art.

C. Analysis of the Expression Level and Expression Site of the Gene of the Present Invention in Plant Issues

The expression level and expression site of a gene in a transgenic plant into which the gene of the present invention has been introduced can be analyzed by extracting RNA from cells and tissues of the plant and detecting the mRNA of the introduced gene by RT-PCR or Northern analysis, which are conventional methods in the art. Alternatively, the expression level and expression site can be analyzed directly by Western blotting or the like of the gene product of the present invention using an antibody against the above product.

D. Changes in the mRNA Levels of Various Genes in a Transgenic Plant into which the Gene of the Present Invention has been Introduced

It is possible to identify by Northern hybridization those genes whose expression levels are believed to have been changed as a result of the action of the transcription factor of the present invention in a transgenic plant into which the gene of the present invention has been introduced.

For example, plants grown hydroponically or the like are given environmental stress for a specific period of time (e.g. 1 to 2 weeks). Examples of environmental stresses include low temperature, dehydration, and salt stresses. For example, dehydration stress may be given by uprooting the plant from the hydroponic medium and drying it on a filter paper for 10 minutes to 24 hours. Low temperature stress may be given by retaining the plant at 15 to −4° C. for 10 minutes to 24 hours. Salt stress can be given by, for example, replacing the hydroponic solution with a 50 to 500 mM NaCl solution and retaining the plant for 10 minutes to 24 hours.

Total RNAs are respectively prepared from a control plant, which was given no stress, and from the plant, which was given environmental stress, and the resultant total RNAs are subjected to electrophoresis. The expression patterns can be analyzed by Northern hybridization using the probe of the gene to be observed.

E. Evaluation of the Tolerance of the Transgenic Plant to Environmental Stresses

The tolerance to environmental stresses of the transgenic plant into which the gene of the present invention has been introduced can be evaluated by setting the transgenic plant in a pot containing a soil comprising vermiculite, perlite and the like, exposing the plant to various environmental stresses, and examining the survival of the plant. Environmental stresses include low temperature, dehydration, and salt stresses. For example, tolerance to dehydration stress can be evaluated by leaving the plant without watering for 2 to 4 weeks and then examining the survival. Tolerance to low temperature and freezing stresses can be evaluated by leaving the plant at 15 to −10° C. for 1 to 10 days, growing it at 20 to 35° C. for 2 days to 3 weeks, and then examining its survival ratio. Tolerance to salt stress can be evaluated by, for example, leaving the plant in 100 to 600 mM NaCl for 1 hour to 7 days, growing it at 20 to 35° C. for 1 to 3 weeks, and then examining its survival rate.

F. Determination of Stress Levels in Plants

The transcription of the gene according to the present invention is activated by low temperature stress, dehydration stress, or salt stress. Therefore, determination of the transcription level of the gene of the present invention enables the assessment of the stress level such as low temperature, dehydration, or salt stress which the plant is subjected to.

The transcription level of the gene according to the present invention can be determined by, for example, RNA gel blot analysis or quantitative PCR. A probe to be used in RNA gel blot analysis can be produced in accordance with any conventional method based on the gene according to the present invention and/or a 100-1000 bp region comprising specific sequence adjacent to the gene. A primer to be used in quantitative PCR can be prepared by any conventional method based on the sequence in the region encoding the gene of the present invention or the region adjacent thereto.

The above-described probe or primer may be used in a kit for determining the transcription level of the gene according to the present invention.

G. Others

In addition, the protein according to the present invention can be utilized by producing an antibody against the protein. The antibody may be a polyclonal or monoclonal antibody. The method for producing an antibody is not particularly limited, and it can be carried out in accordance with any conventional method [see, for example, Sambrook, J et al., Molecular Cloning, Cold Spring Harbor Laboratory Press (1989)]. The antibody can be utilized in, for example, the detection of the protein of interest by Western blotting or immunoprecipitation.

EXAMPLES

The present invention is described in more detail with reference to the following examples, however, the scope of the present invention is not limited to these.

Example 1 Screening of Rice OsDREB Gene

1. Homology Search Against Database

On the basis of the full-length amino acid sequences of DREB1A, DREB1B, DREB1C, DREB2A, and DREB2B genes as shown below, homology search was carried out by BLAST against the database of rice DNA in GenBank.

As a result, four types of genes were discovered: 1 type (OsDREB1B) from EST data, 2 types (OsDREB1C and OsDREB1D) from genome sequence data in terms of the DREB1-homologous gene, and 1 type (OsDREB2A) from EST data in terms of the DREB2-homologous gene.

2. Search of cDNA Library

A. Preparation of cDNA Library

Rice seeds (Nipponbare) were grown hydroponically using distilled water under dark conditions at 25° C. for 15 days. The resulting plant bodies were treated at 4° C. for 2 hours or 24 hours, uprooted from the incubator and dried on a filter paper for 10 hours, or treated with 250 mM NaCl for 10 hours, followed by freezing with liquid nitrogen. Total RNA was extracted from the frozen sample using the guanidine thiocyanate-cesium chloride method, and mRNA was prepared using the Oligo(dt)-cellulose column. cDNA was synthesized using the resultant mRNA as a template and using HybriZAP-2.1 two-hybrid cDNA Gigapack cloning kit (Stratagene) and the cDNA was inserted and cloned in the EcoRI-XhoI cleavage site of HybriZAP-2.1 phagemid vector. This phagemid DNA was packaged using Gigapack III Gold packaging extract (Stratagene). The obtained lambda phage particles containing cDNA were used to infect host E. coli, which were then amplified, and these were subsequently recovered. The resulting phage suspension was then stored.

B. Search Using Probe

The sequence containing the ERF/AP2 domain and a conserved region on the N-terminal side of the OsDREB1D genome sequence obtained in (1) was amplified by PCR to produce a probe. The cDNA library, which was prepared in (1), was searched using this probe. As a result, a new DREB1-homologous cDNA (OsDREB1A) was obtained.

The EST clone corresponding to OsDREB1B was provided by the Rice Genome Research Projects. To amplify the full-length of the protein-coding region, OsDREB1C and OsDREB1D were subjected to PCR using a primer that was designed on the basis of predictions from a genome sequence of a transcription initiation site and a termination codon.

The probe for searching the full-length cDNA of OsDREB2A was produced based on the sequence of EST, thereby searching the cDNA library. Since the resultant cDNA clone was predicted to be of an incomplete length, 5′RACE was carried out using DNA prepared from the cDNA library as a template to determine the full-length sequence. Based on this sequence, a primer for amplifying a full-length gene was designed and the full-length gene was obtained by RT-PCR.

C. Nucleotide Sequencing

The nucleotide sequence of the cDNA of the resultant DREB-homologous gene was determined using 377 DNA sequencer (Perkin-Elmer). Further, the ORF was analyzed to determine all the amino acid sequences.

3. Results:

As a result, nucleotide sequences for 5 types of OsDREB genes and corresponding amino acid sequences of OsDREB proteins were identified. As the DREB protein derived from Arabidopsis thaliana, all the OsDREB proteins comprised regions which were deduced to be: the ERF/AP2 DNA binding domain at the center, a nuclear localization signal at the N-terminus, and an acidic activation domain at the C-terminus (FIG. 1).

In FIG. 2 and FIG. 3, amino acid sequences of the DREB-homologous proteins from various plants were compared to one another to find highly conserved sequences. Outline letters on colored backgrounds represent highly conserved regions.

Sequence numbers of nucleotide sequences and amino acid sequences of each OsDREB are as follows:

OsDREB1A: nucleotide sequence (SEQ ID NO: 1), amino acid sequence (SEQ ID NO: 2);

OsDREB1B: nucleotide sequence (SEQ ID NO: 3), amino acid sequence (SEQ ID NO: 4);

OsDREB1C: nucleotide sequence (SEQ ID NO: 5), amino acid sequence (SEQ ID NO: 6);

OsDREB1D: nucleotide sequence (SEQ ID NO: 7), amino acid sequence (SEQ ID NO: 8);

OsDREB2A: nucleotide sequence (SEQ ID NO: 9), amino acid sequence (SEQ ID NO: 10).

Example 2 Analysis of Ability of OsDREB Proteins to Bind to DRE

A fusion protein between glutathione-S-transferase (GST) and proteins of OsDREB1A and OsDREB2A was prepared using E. coli. The resulting protein was then assessed by gel shift assay to inspect the proteins' binding abilities to DRE.

The 477 bp DNA fragment located from position 69 to position 545 of the nucleotide sequence of OsDREB1A cDNA or the 489 bp DNA fragment located from position 334 to position 822 of the nucleotide sequence of OsDREB2A cDNA was amplified by PCR. Then, the amplified fragment was ligated to the EcoRI-XhoI site of plasmid pGEX-4T-1 (Pharmacia). After the introduction of this plasmid into E. Coli XL1-Blue MRF′, the E. coli was cultured in 500 ml of 2×YT medium (Molecular Cloning (1982), Cold Spring Harbor Laboratory Press). To this culture, 0.1 mM isopropyl β-D-thiogalactoside, which activates the promoter of plasmid pGEX-4T-1, was added to induce the synthesis of a fusion protein of OsDREB1A and GST.

The E. coli into which the protein had been induced was suspended in 18 ml of buffer (10 mM Tris-HCl, pH 8.0, 0.1 mM EDTA, 5 mM MgCl₂, 400 mM NaCl, 5% glycerol, 0.1 mM phenylmethylsulfonyl fluoride, 0.1 mM dithiothreitol). Then, 1% Triton X-100 and 1 mM EDTA were added thereto. After the cells were disrupted by sonication, the disrupted material was centrifuged at 20,000 g for 1 hour. Then, the protein was purified from the supernatant using glutathione-Sepharose (Pharmacia). The resultant fusion protein was incubated at room temperature for 20 minutes using the DRE sequence-containing 75 bp DNA fragment (SEQ ID NO: 16) labeled with ³²P as a probe. This mixture was electrophoresed using 5% polyacrylamide containing 0.25×Tris-borate-EDTA at 120 V for 90 minutes. As a result of this gel shift assay, those bands which migrated to the back were detected. When the DNA fragment containing the mutated DRE sequence was used as a probe, such bands were not detected. Thus, it became evident that OsDREB1A and OsDREB2A proteins specifically bind to the DRE sequence (FIG. 4).

Example 3 Preparation of Transformant (Transgenic Plant)

1. Construction of Plant Plasmid

A. Preparation of OsDREB1A Gene Fragment

The 717 bp DNA fragment located from position 69 to position 785 of the nucleotide sequence of cDNA of the OsDREB1A gene was amplified by PCR using the following primers. Thereafter, the amplified fragment was ligated to the BamHI cleavage site of the vector pBluescript SK(−) (Stratagene) to obtain the recombinant plasmid pSKOsDREB1A. This pSKOsDREB1A was cleaved with BamHI to obtain approximately 700 bp DNA fragment containing OsDREB1A gene. Forward: 5′-GGGGATCCATGTGCGGGATCAAGCAGGAGAT (SEQ ID NO: 17) G-3′ Reverse: 5′-GGGGATCCCTAGTAGCTCCAGAGTGGGA (SEQ ID NO: 18) C-3′

B. Preparation of pBE2113Not, G-ubi, G35S-ShΔ

pBE2113Not (Plant Cell 10: 1391-1406(1998)), G-ubi, and G35S-ShΔ were used as plasmids having promoter DNA. G-ubi and G35S-ShΔ were prepared as follows. At the outset, pBIG plasmid (Nucleic Acids Research 18: 203 (1990)) was cleaved with BamHI, blunt-ended and ligated to delete the BamHI cleavage site. Thereafter the plasmid was cleaved with HindIII and EcoRI. The resultant fragment and an approximately 1.2 kb fragment, which was obtained by cleavage of pBE2113Not plasmid in the same manner, were ligated to each other, thereby preparing pBIG2113Not plasmid.

Subsequently, pBIG2113Not was cleaved with HindIII and BamHI and ligated to a fragment of rd29A promoter (approximately 0.9 kb, Nature Biotechnology 17: 287-291 (1999)), which was cleaved in the same manner, thereby preparing pBIG29APHSNot plasmid. Further, this pBIG29APHSNot plasmid was cleaved with HindIII and SalI and then ligated to a fragment of the ubiquitin gene (Ubi-1) promoter (approximately 2.0 kb, Plant Molecular Biology 18: 675-689 (1992)) of maize, which was cleaved in the same manner, or to a fragment (approximately 1.6 kb, Proceeding National Academy of Science USA 96: 15348-15353 (1999)) containing CaMV 35S promoter of p35S-shΔ-stop and a part of the intron of a sucrose synthase gene (Sh1) of maize. Thus, G-ubi plasmid or G35S-shΔ plasmid was prepared. pBE2113Not, G-ubi, and G35S-shΔ described above were respectively cleaved with BamHI and ligated to the OsDREB1A gene fragment using Ligation High (Toyobo Co., Ltd.). E. coli DH5α was transformed using the thus obtained ligation product. After the transformant was cultured, plasmid pBE35S:OsDREB1A, G-ubi: OsDREB1A, and G35S-ShΔ: OsDREB1A were respectively purified therefrom. Subsequently, the nucleotide sequences thereof were determined, and those having OsDREB1A gene bound in the sense direction were selected.

C. Introduction into Agrobacterium

The plasmid pBE35S: OsDREB1A-containing E. coli DH5a, helper plasmid pRK2013-containing E. coli HB101, and Agrobacterium C58 were mixed and cultured on LB agar medium at 28° C. for 24 hours. Generated colonies were scraped off and suspended in 1 ml of LB medium. This suspension (10 μl) was coated on LB agar medium containing 100 mg/l rifampicilin and 20 mg/l kanamycin and cultured at 28° C. for 2 days, thereby obtaining zygote Agrobacterium C58 (pBE35S: OsDREB1A). By electroporation, the plasmid G-ubi: OsDREB1A and plasmid G35S-ShΔ: OsDREB1A were separately introduced into Agrobacterium EHA105, which were then washed with 10% glycerol after culturing. Thus, Agrobacterium EHA105 (G-ubi:OsDREB1A) and Agrobacterium EHA105 (G35S-shΔ: OsDREB1A) were prepared.

2. Gene Introduction into Arabidopsis thaliana by Agrobacterium Infection

The zygote was cultured in 10 ml of LB medium containing 100 mg/l rifampicilin and 20 mg/l kanamycin at 28° C. for 24 hours. Subsequently, this culture fluid was added to 500 ml of LB medium and cultured for 24 hours. The resultant culture fluid was centrifuged to remove the medium and suspended in 500 ml of buffer for infection (2.3 g of Murashige and Skoog Plant Salt Mixture (Nihon Pharmaceutical Co., Ltd), 1 ml of Gamborg's vitamin solution, 50 g of sucrose, 200 μl of L-77 (Nippon Unicar Co., Ltd.), and 10 μg of 6-benzylaminopurine, per liter).

On the other hand, 4 to 5 Arabidopsis thaliana plant bodies were grown in 9 cm pots containing soil composed of equivalent portions of vermiculite and perlite, for 6 weeks. Then, the Arabidopsis thaliana plant body was directly immersed in the Agrobacterium suspension of the Agrobacterium C58 (pBI35S: OsDREB1A) and placed in a desiccator, which was sucked with a vacuum pump to reduce the pressure to 650 mmHg and then left to stand for 10 min. Subsequently, the plant pot was transferred to a tray and covered with a wrap to maintain humidity. On the next day, the wrap was removed. Thereafter, the plant was grown uncovered to produce seeds. After sterilization in an aqueous solution of sodium hypochlorite, the seeds were sown on an agar medium for selection (MS medium supplemented with 100 mg/l vancomycin and 30 mg/l kanamycin). Arabidopsis thaliana grown on this medium were transferred to pots to obtain seeds of the transformed plant.

3. Gene Introduction into Rice by Agrobacterium Infection

Rice seeds were immersed in 70% ethanol for 1 minute and sterilized by immersion into 2% sodium hypochlorite for 1 hour. The sterilized seeds were then washed with sterilized water, and 9 grains each of the seeds were sowed onto a plate of N₆D solid medium (3.98 g of CHU[N₆] Basal Salt Mixture (Sigma), 30 g of sucrose, 100 mg of myo-inositol, 300 mg of casamino acid, 2,878 mg of L-proline, 2 mg of glycine, 0.5 mg of nicotinic acid, 0.5 mg of pyridoxine hydrochloride, 1 mg of thiamine hydrochloride, 2 mg of 2,4-D, and 4 g of Gelrite, per liter, pH 5.8), followed by culturing for 24 days. Thus, callus was induced. The calluses formed from approximately 20 grains of the seeds were transferred to new N₆D solid medium, followed by culturing for additional three days.

Separately, Agrobacterium EHA105 (G-ubi: OsDREB1A) and Agrobacterium EHA105 (G35S-ShD: OsDREB1A) were cultured in 5 ml of YEP medium containing 100 mg/l rifampicilin and 20 mg/l kanamycin (10 g of Bacto peptone, 10 g of Bacto yeast extract, 5 g of NaCl, and 406 mg of MgCl₂.6H₂O, per liter, pH 7.2) at 28° C. for 24 hours. This Agrobacterium was diluted with AAM medium containing 20 mg/l acetosyringon (10 mg of MnSO₄.5H₂O, 3 mg of H₃BO₃, 2 mg of ZnSO₄.7H₂O, 250 μg of Na₂MoO₄.2H₂O, 25 μg of CuSO₄.5H₂O, 25 μg of CoCl₂.6H₂O, 750 μg of KI, 150 mg of CaCl₂.2H₂O, 250 mg of MgSO₄.7H₂O, 40 mg of Fe-EDTA, 150 mg of NaH₂PO₄.2H₂O, 1 mg of nicotinic acid, 10 mg of thiamine hydrochloride, 1 mg of pyridoxine hydrochloride, 100 mg of myo-inositol, 176.7 mg of L-arginine, 7.5 mg of glycine, 900 mg of L-glutamine, 300 mg of aspartic acid, and 3 g of KCl, per liter, pH 5.2) to bring O.D.₆₆₀ to 0.1. Thus, 20 ml of Agrobacterium suspension was prepared.

Subsequently, to the callus, which was cultured for 3 days, the Agrobacterium suspension was added and then mixed for 1 minute. Thereafter, this callus was placed on a sterilized paper towel to remove excess Agrobacterium suspension and then cultured on 2N₆-AS solid medium, on which the sterilized filter paper was placed, (3.98 g of CHU[N₆] Basal Salt Mixture, 30 g of sucrose, 10 g of glucose, 100 mg of myo-inositol, 300 mg of casamino acid, 2 mg of glycine, 0.5 mg of nicotinic acid, 0.5 mg of pyridoxine hydrochloride, 1 mg of thiamine hydrochloride, 2 mg of 2,4-D, 10 mg of acetosyringon, and 4 g of Gellite, per liter, pH 5.2) at 25° C. for 3 days in the dark. After culturing for 3 days, the culture product was thoroughly washed with an aqueous solution of 3% sucrose containing 500 mg/l carbenicillin until the product did not whiten. The washed culture product was further cultured on N₆D solid medium containing 500 mg/l carbenicillin and 10 mg/l hygromycin for 1 week. Thereafter, the resulting culture product was transferred onto a N₆D solid medium containing 500 mg/l carbenicillin and 50 mg/l hygromycin and cultured for 18 days. Furthermore, the callus was transferred to a regeneration medium (4.6 g of Murashige and Skoog Plant Salt Mixture (Nihon Pharmaceutical Co., Ltd), 30 g of sucrose, 30 g of sorbitol, 2 g of casamino acid, 100 mg of myo-inositol, 2 mg of glycine, 0.5 mg of nicotinic acid, 0.5 mg of pyridoxine hydrochloride, 0.1 mg of thiamine hydrochloride, 0.2 mg of NAA, 2 mg of kinetin, 250 mg of carbenicillin, 50 mg of hygromycin, and 8 g of agarose, per liter, pH 5.8). The product was transferred to a new medium every week and regeneration. Those having buds grown to approximately 1 cm were transferred to a hormone-free medium (4.6 g of Murashige and Skoog Plant Salt Mixture (Nihon Pharmaceutical Co., Ltd), 30 g of sucrose, 2 mg of glycine, 0.5 mg of nicotinic acid, 0.5 mg of pyridoxine hydrochloride, 0.1 mg of thiamine hydrochloride, 50 mg of hygromycin, and 2.5 g of Gellite, per liter, pH 5.8). Plant bodies, which have grown to approximately 8 cm on the hormone-free medium, were transferred to a pot containing synthetic particulate potting soil (Bonsol No. 1, Sumitomo Chemical Co., Ltd.) to allow the transgenic plant to produce seeds.

Example 4 Analysis of the Transcription Activating Mechanism Using Rice Protoplast

As shown in FIG. 5, to construct an effector plasmid, OsDREB1A cDNA, OsDREB2A cDNA, DREB1A cDNA, and DREB2A cDNA were positioned downstream of the CaMV35S promoter and the intron sequence of sucrose synthetase of maize and ligated to pBI221 plasmid (Clontech). Separately, a reporter plasmid was constructed in which a 75 bp DNA fragment containing DRE of rd-29A promoter was repeatedly inserted twice upstream of the minimal promoter-61 rd29A and a GUS reporter gene.

Subsequently, these two plasmids were introduced into the rice protoplast and GUS activity was then determined based on changes in fluorescence intensity caused by decomposition of 4-methylumbelliferyl-β-D-glucuronide. The fusion gene of CaMV35S promoter-LUC was simultaneously introduced as a standard for the introduction efficiency in each experiment. As a result, OsDREB1A and OsDREB2A genes were found to activate transcription through DRE.

Example 5 Analysis of Expression of OsDREB Gene in Transformant

1. Analysis of Expression of OsDREB Gene in Nontransformant

Expression properties of OsDREB1A, OsDREB1B, OsDREB1C, and OsDREB2A genes in wild-type rice were analyzed by Northern hybridization. Rice was cultured hydroponically at 25° C. under insolation conditions of 16 hours during the day and 8 hours at night for 17 days. Abscisic acid, dehydration, low temperature, salt (NaCl), lesion, and water stresses were separately applied to the plant body. Sampling was accomplished on stress-applied rice every 0, 10, 20, 40, 60 minutes, 2, 5, 10, and 24 hours.

Each stress was applied to the rice as follows: abscisic acid stress was applied by immersing in a solution containing 100 μM ABA; dehydration stress was applied by drying on a filter paper; low temperature stress was applied by transferring to an incubator which was cooled at 4° C.; salt (NaCl) stress was applied by immersing in an aqueous solution containing 250 mM NaCl; lesion stress was applied by slitting up 8 to 10 cm-high leaves; and water stress was applied by immersing in pure water. Total RNA was separately prepared from a control plant which was given no stress and the plant which was given stress. The RNAs were then subjected to electrophoresis. Thus, the expression of each gene was observed by the Northern method. The result is shown in FIG. 6.

From analysis, the expression of the OsDREB1A gene and that of the OsDREB1B gene were respectively induced mainly by low temperature stress. In contrast, the expression of OsDREB2A was induced mainly by dehydration and salt stresses. Gene expression was constantly observed in OsDREB1C.

2. Analysis of OsDREB Gene Expression in Transformed Arabidopsis thaliana

In the same manner as in Example 3, transformants having OsDREB1A, OsDREB1D, and OsDREB2A genes introduced into Arabidopsis thaliana were prepared. The mRNA level of the transformant-introduced genes OsDREB1A, OsDREB1D, and OsDREB2A and that of the genes, the expression of which was considered to be altered by the introduced genes, were analyzed by the Northern method. Specifically, partial fragments of rd29A gene, cor15a gene, kin1 gene, and erd10 gene were used as probes (rd29A: SEQ ID NO: 19, cor15a: SEQ ID NO: 20, kin1 SEQ ID NO: 21, erd10: SEQ ID NO: 22), and the mRNA levels were analyzed. In addition to the transformant, transformed Arabidopsis thaliana having pBI121 plasmid (Clontech) containing no DREB-homologous gene introduced therein was used as a control to compare the gene expressions.

Approximately 1 g of plant bodies grown on GM agar medium for 3 weeks was exposed to dehydration stress and low temperature stress. Dehydration stress was applied by uprooting the plant from the agar medium and drying it on a petri dish for 5 hours. Low temperature stress was applied by incubating the plant at 4° C. for 5 hours. Total RNA was prepared separately from control plants which are given no stress and plants which were given the dehydration and low temperature stresses. The resultant total RNAs were subjected to electrophoresis. Then, gene expressions were assessed by the Northern method.

Generally, genes are introduced into the genome of a transformant in a similar manner, however, due to differences in the locations on the genome and thereby the expression of the introduced genes vary. This phenomenon is called “position effect.” In this experiment, by assaying transformants with DNA fragments from the introduced gene as a probe by the Northern method, those transformants in which the introduced gene was expressed more highly could be selected. Also, by using a DNA fragment of the gene, which could be involved in the stress tolerance, as a probe, OsDREB1A was introduced. Thus, the gene having a varied level of mRNA was identified. The result is shown in FIG. 7.

As a result, the gene having GCCGAC in the promoter was induced more strongly than the gene having ACCGAG In the group of stress tolerant genes of monocotyledonous plants, those having GCCGAC as the DRE sequence exist in a larger amount than those having ACCGAG. Accordingly, it is suggested that, in these monocotyledonous plants, the OsDREB genes allowed the stress tolerant genes to express more efficiently than the DREB genes.

3. Analysis of OsDREB Gene Expression in Transformed Rice

In the same manner as in Example 3, transformants having OsDREB1A, OsDREB1B, and DREB1C genes of Arabidopsis thaliana introduced into rice were prepared. The mRNA level of the transformant-introduced genes OsDREB1A, OsDREB1B, and DREB1C of Arabidopsis thaliana and that of the gene, the expression of which was considered to be altered by the introduced genes, were analyzed by the Northern method. Specifically, partial fragments of OsDREB1A gene, OsDREB1B gene, DREB1C gene, lip9 gene, Wsi724 gene, and salT gene were used as probes (OsDREB1A: SEQ ID NO: 23, OsDREB1B: SEQ ID NO: 24, DREB1C: SEQ ID NO: 25, lip9: SEQ ID NO: 26, Wsi724: SEQ ID NO: 27, salT: SEQ ID NO: 28), and the expression levels of mRNA were analyzed. In the analysis, in addition to the transformant, transformed rice having G-ubi that contained no DREB-homologous gene introduced therein was used as a control in order to compare gene expressions.

Selection was carried out in a 0.1% Benlate solution containing 30 mg/ml hygromycin for 5 days. Thereafter, the plant was transferred to a pot containing Bonsol No. 1 and was grown for 12 days. Approximately 2 g of the grown plant was subjected to salt (NaCl) and low temperature stresses. Salt stress was applied by uprooting plant body from the soil and immersing in 250 mM NaCl in a test tube for 5 hours. Low temperature stress was applied by incubating the plant body at 4° C. for 5 hours. Total RNAs were separately prepared from a control plant which was given no stress and the plant which was given salt and low temperature stresses, and then subjected to electrophoresis. Thus, the expression of each gene was observed by the Northern method in the same manner as in (2). The result is shown in FIG. 8.

As a result, in the transformed rice having OsDREB1A, OsDREB1B, and DREB1C genes introduced therein, the expression of the lip9 gene having the DRE sequence in the promoter region was induced while the expression of the salT gene having no DRE sequence in the promoter region was not induced. Also, the expression of the Wsi724 gene, the expression of which in the promoter region was not identified but deduced to be a target of OsDREB, based on the expression pattern when stress was applied (dehydration, salt, low temperature inducible, induction by low temperature is slower than that by dehydration and salt), was induced in these transformants.

Example 6 Influences of OsDREB Genes on Arabidopsis thaliana Stress Tolerances

In the same manner as in Example 3, transformants having OsDREB1A and DREB1A genes introduced into Arabidopsis thaliana were prepared. As a control, Arabidopsis thaliana, which was transformed with pBI121 containing no DREB-homologous gene, was prepared. Each tolerance experiment was carried out under following conditions.

1. NaCl Tolerance

NaCl tolerance was inspected as follows. Arabidopsis thaliana, which was grown in GM medium for 3 weeks, was immersed in an aqueous solution of 600 mM NaCl for 2 hours, followed by washing. Thereafter, the plant body was transferred into a pot containing Professional potting soil and cultured for 3 weeks, and its survival rate was assessed.

2. Dehydration Tolerance

Dehydration tolerance was investigated as follows. Arabidopsis thaliana, which was grown in GM medium for 3 weeks, was transferred in a pot containing soil composed of equivalent portions of vermiculete and perlite, and cultured for 1 week, and water supply was then stopped. After culturing for 2 weeks, its survival ratio was assessed.

3. Freezing Tolerance

Freezing tolerance was investigated as follows. Arabidopsis thaliana, which was grown in GM medium for 3 weeks, was transferred into a pot containing Professional potting soil and cultured for 1 week. Thereafter, the plant body was placed at −6° C. for 36 hours and then cultured at 22° C. for 5 days. Its survival ratio was then assessed.

In the experiment for inspecting salt stress tolerance, the survival ratio was 12% for the control and 55% or 65% for the OsDREB1A-introduced plant. As for the DREB1A-introduced plant, the survival ratio was 68% for 35S: DREB1A and 90% for 29A: DREB1A. This indicates that the OsDREB genes also improve stress tolerance in dicotyledonous plants (FIG. 9).

All publications, patents and patent applications cited herein are incorporated herein by reference in their entirety.

FREE TEXT OF SEQUENCE LISTING

-   -   SEQ ID NO: 16; probe     -   SEQ ID NO: 17; primer     -   SEQ ID NO: 18; primer     -   SEQ ID NO: 19; probe for rd29a     -   SEQ ID NO: 20; probe for cor15a     -   SEQ ID NO: 21; probe for kin1     -   SEQ ID NO: 22; probe for erd10     -   SEQ ID NO: 23; probe for OsDREB1A     -   SEQ ID NO: 24; probe for OsDREB1B     -   SEQ ID NO: 25; probe for DREB1C     -   SEQ ID NO: 26; probe for lip9     -   SEQ ID NO: 27; probe for Wsi724     -   SEQ ID NO: 28; probe for salT 

1. An isolated nucleic acid molecule which comprises the nucleotide sequence as shown in SEQ ID NO: 3, encoding the OsDREB1B protein.
 2. (canceled)
 3. A recombinant vector comprising the nucleic acid molecule according to claim
 1. 4. An isolated host cell transformed with the recombinant vector according to claim
 3. 5. A transgenic plant transformed with the recombinant vector according to claim
 3. 6. A method for producing the OsDREB1B protein which regulates the transcription of genes located downstream of a stress responsive element, wherein the transgenic plant according to claim 5 is cultured in a medium and said protein is recovered from the resultant culture product.
 7. The transgenic plant according to claim 5, wherein the plant is a monocotyledonous plant.
 8. A method for producing the OsDREB1B protein which regulates the transcription of genes located downstream of a stress responsive element, wherein the transgenic plant according to claim 7 is cultured in a medium and said protein is recovered from the resultant culture product.
 9. (canceled)
 10. A method for improving the stress tolerance of plants by introducing the nucleic acid molecule according to claim 1 into the plants.
 11. An isolated nucleic acid molecule encoding a protein which comprises the amino acid sequence as shown in SEQ ID NO:
 4. 12. (canceled)
 13. A recombinant vector comprising the nucleic acid molecule according to claim
 11. 14. An isolated host cell transformed with the recombinant vector according to claim
 13. 15. A transgenic plant transformed with the recombinant vector according to claim
 13. 16. A method for producing the OsDREB1B protein which regulates the transcription of genes located downstream of a stress responsive element, wherein the transgenic plant according to claim 15 is cultured in a medium and said protein is recovered from the resultant culture product.
 17. The transgenic plant according to claim 15, wherein the plant is a monocotyledonous plant.
 18. A method for producing the OsDREB1B protein which regulates the transcription of genes located downstream of a stress responsive element, wherein the transgenic plant according to claim 17 is cultured in a medium and said protein is recovered from the resultant culture product.
 19. (canceled)
 20. A method for improving the stress tolerance of plants by introducing the nucleic acid molecule according to claim 11 into the plants.
 21. The following recombinant protein (c) or (d): (c) a protein which comprises the amino acid sequence as shown in SEQ ID NO: 2; or (d) a protein which comprises the amino acid sequence having deletion, substitution, or addition of one or several amino acids in the amino acid sequence as shown in SEQ ID NO: 2 and which regulates the transcription of genes located downstream of a stress responsive element.
 22. The protein according to claim 21, wherein the stress is dehydration stress, low temperature stress or salt stress.
 23. A recombinant vector comprising the nucleic acid molecule according to claim 1, operably linked downstream of a stress responsive promoter.
 24. An isolated host cell transformed with the recombinant vector according to claim
 23. 25. A transgenic plant transformed with the recombinant vector according to claim 23, wherein the plant is a monocotyledonous plant.
 26. A recombinant vector comprising the nucleic acid molecule according to claim 11, operably linked downstream of a stress responsive promoter.
 27. An isolated host cell transformed with the recombinant vector according to claim
 26. 28. A transgenic plant transformed with the recombinant vector according to claim 26, wherein the plant is a monocotyledonous plant. 